The use of cluster bombs will be given backing under plans being drawn up at an international treaty conference, according to opponents of the weapons which maim and kill civilians, notably children, long after they have been dropped.

The bombs are banned under the 2008 convention on cluster munitions, which was adopted by the UK and more than 100 other countries. But the US refused to sign and is pressing for a protocol to be added to the UN convention on certain conventional weapons (CCW) to provide legal cover for the use of cluster munitions.

The US is being supported by other cluster bomb manufacturers – including Russia, China, Israel, India and Pakistan – at negotiations due to end in Geneva on Friday. The move is also backed by a number of signatories to the 2008 convention, including France, Germany, Italy, Portugal and Australia, conference observers said.

Amnesty International, Oxfam and Article 36, a group which co-ordinates opposition to such weapons systems, said humanitarian concerns were being ignored at the UN-sponsored talks and that they will on Wednesday call on Britain to resist US attempts to sanction what they described as a "licence to kill" with cluster bombs.

The Foreign Office said Britain was committed to the cluster bomb convention and would not accept the proposed protocol unless it provided clear humanitarian benefits, an official said. "The current draft protocol text does not do this."

The US and its supporters argue that the proposed CCW protocol would permit the use of cluster bombs manufactured after 1980 and that these had a less than 1% failure rate. Opponents say that cut-off point is arbitrary, that most bombs produced before 1980 are unusable and that modern cluster munitions have failure rates much higher than the manufacturers claim.

Thomas Nash, director of Article 36, said the draft protocol gave legal cover for the use of cluster bombs and described any move towards that as "mind boggling".

"It will allow some of the worst cluster bombs ever made to be used," he added, referring to the BLU-97 combined effects bomb which contains bomblets that, as they fall, fragment and can turn into an incendiary weapon.

The weapon has been widely used by US forces with a reported failure rate of more than 30%. The unexploded bomblets have the appearance of yellow drink containers and are attractive, but lethal, when handled by civilians.

"We need the UK to speak up," said Nash, adding that the British delegation in Geneva had so far said nothing during the negotiations.

Anna Macdonald, of Oxfam International, said: "We will need more leadership from ministers this week to resist US pressure."

Amnesty's Oliver Sprague said: "The UK has quite rightly championed the total ban on cluster munitions. It must not now support cynical attempts by the US to undermine efforts to eradicate these deadly and indiscriminate weapons by agreeing to a new legal standard."

Other weapons, including white phosphorus, are on the agenda in Geneva.

A Foreign Office spokesman said: "The UK is committed to ridding the world of cluster munitions … We will take a view on the protocol at the end of these negotiations. We have been clear that we will not sign up to anything which would undermine [the convention on cluster munitions] or dilute our obligations under it."